Wireless communication systems are widely deployed to provide various types of communication content such as voice, video, packet data, messaging, broadcast, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., time, frequency, and power). Examples of such multiple-access systems include code-division multiple access (CDMA) systems, time-division multiple access (TDMA) systems, frequency-division multiple access (FDMA) systems, and orthogonal frequency-division multiple access (OFDMA) systems.
By way of example, a wireless multiple-access communication system may include a number of base stations, each simultaneously supporting communication for multiple wireless devices (e.g., mobile phones or tablet computers). A base station may communicate with wireless devices on downlink channels (e.g., for transmissions from the base station to the wireless devices) and uplink channels (e.g., for transmissions from the wireless devices to the base station).
Some modes of communication may enable communications with a wireless device over different radio frequency spectrum bands (e.g., a licensed radio frequency spectrum band or an unlicensed radio frequency spectrum band) of a cellular network. With increasing data traffic in cellular networks that use a licensed radio frequency spectrum band, offloading of at least some data traffic to an unlicensed radio frequency spectrum band may provide a wireless communications system operator with opportunities for enhanced data transmission capacity. An unlicensed radio frequency spectrum band may provide one or more contention-based communications channels in which a transmitting apparatus may, in some examples, perform a listen before talk (LBT) procedure to contend for access to the unlicensed radio frequency spectrum band. An LBT procedure may include performing a clear channel assessment (CCA) to determine whether a channel of the unlicensed radio frequency spectrum band is available. When it is determined that the channel of the unlicensed radio frequency spectrum band is unavailable (e.g., because another device is already using the channel of the unlicensed radio frequency spectrum band), a CCA may be performed for the channel again at a later time. When it is determined that the channel of the unlicensed radio frequency spectrum band is available, or clear, a channel usage beacon signal (CUBS) may be transmitted over the channel, to reserve the channel until a downlink transmission or uplink transmission may be made over the channel.